1. Field of the Invention
The present invention relates to a process for producing an SOI (Silicon On Insulator) substrate in which an SOI layer is manufactured on an oxide film by use of hydrogen ion implantation technology. Additionally, the present invention relates to a process for the regeneration of a layer transferred wafer in the process for producing an SOI.
2. Description of the Related Art
Conventionally, a process for producing an SOI substrate has been known in which after an oxide film is formed on the surface of a first silicon substrate, hydrogen ions of a high concentration are implanted into the inside thereof, and an annealing process is performed at a high temperature. Additionally, a hydrogen ion implantation area is formed in a predetermined depth from this silicon substrate surface, then a second silicon substrate is laminated onto this first silicon substrate and a laminated body of the two substrates bonded with each other is formed, and this laminated body is heated up to a temperature over 500° C. and the first silicon substrate is separated from the second silicon substrate at the hydrogen ion implantation area, and thereby a semiconductor SOI layer is formed on the surface of the second silicon substrate. (For example, Japanese Unexamined Patent Application Publication No. 5-211128.) According to this method, it is possible to produce an SOI substrate having the second silicon substrate and the oxide film formed on this substrate and functioning as a buried oxide film and the semiconductor SOI layer formed on this oxide film.
Further, when an SOI substrate is produced by such a method as the above, because the first silicon substrate is separated at the hydrogen ion implantation area and the semiconductor SOI layer is formed on the surface of the second silicon substrate, a layer transferred wafer at the side where the first silicon substrate is separated and an SOI layer is not formed is by-produced inevitably. And according to this method for producing an SOI substrate, since this by-produced layer transferred wafer is regenerated and reused as a first silicon substrate once again and thereby a plurality of SOI substrates can be obtained, as a result, it is possible to reduce costs to a great extent. (For example, Japanese Unexamined Patent Application Publication No. 2001-155978.)
Herein, crystal defects of the SOI layer such as an OSF, a COP, an L/D and the like in the SOI layer of an SOI substrate deteriorate device characteristics, and so, it is necessary to reduce those crystal defects. Accordingly, it is desired that the first silicon substrate on which an SOI layer is formed by separation after ion implantation should be a wafer free of such crystal defects. Herein, “OSF” refers to an Oxidation Induced Stacking Fault, “COP” refers to a Crystal Originated Particle, and “L/D” refers to an Interstitial-type Large Dislocation. Specifically, micro defects to become nuclei of this OSF are introduced into a crystal at the growth of the crystal, and this OSF becomes obvious in a thermal oxidation process or the like in producing a semiconductor device and causes failures including increased leak current and the like of a manufactured device. Further, the COP is a pit arising from a crystal that occurs on a wafer surface when a silicon wafer after mirror polishing is cleaned with a mixed liquid of ammonia and hydrogen peroxide. When this wafer is measured with a particle counter, this pit as well as original particles are detected as light scattering defects. This COP causes the deterioration of electric characteristics such as, for example, a Time Dependent Dielectric Breakdown (TDDB) of an oxide film, and a Time Zero Dielectric Breakdown (TZDB) of an oxide film and the like. Further, if there is a COP on a wafer surface, a dump occurs in a device wiring process, leading to a wire disconnection. And in an element separation portion too, the COP causes a leak and the like, decreasing the yield of products. Further, the L/D is also called a dislocation cluster, or a dislocation pit because when a silicon wafer with this defect is dipped in a selective etching liquid made mainly of hydrofluoric acid, a pit is generated. This L/D also causes the deterioration of electric characteristics such as, for example leak characteristics, isolation characteristics and the like.
On the other hand, a defect-free silicon wafer not having such an OSF, COP and L/D is known. (For example, Japanese Unexamined Patent Application Publication No. 11-1393.) This defect-free silicon wafer is a silicon wafer cut out from an ingot made of a perfect area [P] which is a perfect area where there is not an agglomerate of vacancy type point defects and an agglomerate of interstitial silicon type point defects in a silicon single crystal ingot. The perfect area [P] exists between an area [I] where the interstitial silicon type point defects exist dominantly, and an area [V] where the vacancy type point defects exist dominantly in a silicon single crystal ingot. A silicon wafer made of this perfect area [P] is so prepared that when the pulling velocity of an ingot is defined as V (mm/minute), and the temperature gradient in the vertical direction of the ingot at the vicinity of the interface between a silicon melt and the ingot is defined as G (° C./mm), the value of V/G (mm2/minute ° C.) should be determined in order that OSFs that occur in a ring shape at a thermal oxidation process should cease to exist at the central portion of the wafer. Accordingly, it is considered that by producing an SOI substrate with such a defect-free silicon wafer as a first silicon substrate, and reusing a layer transferred wafer by-produced at the production thereof as a first silicon substrate once again, a plurality of SOI substrates free of OSF, COP, and L/D in the SOI layer thereof can be obtained repeatedly.
However, when a wafer free of crystal defects sliced from an ingot not hydrogen doped is used as a first silicon substrate, there is a fear that oxygen existing in solid solution form in the first silicon substrate may become oxygen precipitates in plural times of heat treatment in the production process. Therefore, even if an SOI substrate is produced by use of a wafer free of crystal defects sliced from an ingot not hydrogen doped is used as a first silicon substrate, oxygen precipitates may occur in a layer transferred wafer by-produced at the production thereof, and so, oxygen precipitates may exist in an SOI substrate obtained by reusing the layer transferred wafer as a first silicon substrate, which has been a problem not solved yet in the prior art.